This invention relates generally to the carbonation of alkali metal phenates and relates more particularly to the carbonation of sodium phenate which produces the sodium salt of salicylic acid.
It is well-known that hydroxy aromatic carboxylic acids can be prepared by the reaction of alkali metal phenates with carbon dioxide in the absence of water. See Lindsey et al., Chemical Reviews, 57:583-620 (1957). In this reaction, dry finely divided alkali metal phenate is generally contacted with carbon dioxide at superatmospheric pressures at temperatures of from about 100.degree. C. to about 300.degree. C. over a period of hours to produce the corresponding carboxylic acid derivative. Under these conditions, however, the alkali metal phenate has a tendency to cake or agglomerate into larger particles resulting in inefficient mixing, lower yields of the acid salt product, and localized heating in excess of the desired temperature range leading to the formation of undesirable by-products. Various techniques are employed in the art to avoid this agglomeration, as is illustrated in British Pat. No. 1,205,447.
An inert solvent or suspension system can be employed to disperse the alkali metal phenate into smaller particles which are more efficiently carbonated, as illustrated in U.S. Pat. No. 2,824,892 and British Pat. Nos. 734,622 and 738,359. However, such techniques necessitate the difficult removal and recovery of solvents from the product. This solvent removal/recovery step is relatively expensive and limits the use of such solvents in industrial carbonation processes.
The more common practice in industry has been to employ rotary "ball mills" in the carbonation of sodium phenate to produce salicylic acid. In this method loose pieces of iron or stainless steel are employed inside the rotating mill to grind the aggregate particles into a smaller, more reactive particle size. These "ball mills", however, are difficult to maintain, can contaminate the product with metal fragments, require a large vessel to compensate for the volume occupied by the grinding medium, and are very noisy. Further, the removal of the carbonated product from the mill is difficult and time consuming because the product is not free-flowing. Therefore, it is necessary to rotate the mill during removal of the product.
The foregoing prior art methods for maintaining the relatively higher surface area and higher reactivity of the alkali metal phenate are relatively ineffective or uneconomical. It would be desirable, then, to provide an economical method of carbonation whereby the phenate could be predominantly maintained in a state conducive to carbonation, and whereby the carbonated product can be conveniently removed from the carbonation vessel.
The practice of this invention is useful to efficiently prepare hydroxy aromatic carboxylic acids from phenols. This carbonation process is particularly useful to prepare salicylic acid from phenol.